# ADR-0051: Routing Helper API — `AddressResolver` + `PathRouter`

## Status

Accepted (2026-05-22).

Pins down every public API, argument, return value, and adjacency-graph
selection of the two helper classes (`AddressResolver`, `PathRouter`)
exposed by `policy/routing/router.py`. ADR-0002 defines routing
distance, ordering, and bypass rules, but **the helper API surface
itself** has had no ADR-level coverage.

## First action

### `AddressResolver(graph)`

On construction, caches two pieces of state:

1. `self._node_ids = set(graph.nodes)` — a set of all node ids for
   lookup.
2. `self._hbm_slice_bytes = hbm_total_gb * (1 << 30) // slices_per_cube`
   — derived from `graph.spec.cube.memory_map` (default `48 GB / 8
   slices = 6 GB`). `resolve()` uses this value to decode `pe_id` from
   an HBM PA's `hbm_offset`.

In short, **AddressResolver's first act is "precompute the full set of
node ids and the HBM slice size"**. It does not retain the graph
itself.

### `PathRouter(graph)`

On construction, **builds four separate adjacency graphs in one pass**:

1. `self._adj_all`: every edge (used for component-to-component
   routing).
2. `self._adj`: edges with `kind != "command"` (PE DMA / generic data
   paths).
3. `self._adj_mcpu_dma`: excludes
   `_MCPU_DMA_EXCLUDE = {"pe_internal", "pe_to_router"}` (M_CPU DMA
   must not pass through PE pipeline nodes).
4. `self._adj_local`: excludes the 8-element `_UCIE_KINDS` set (UCIe
   would look like a zero-distance bus to Dijkstra, which would prefer
   it over the mesh — for cube-local routing this must be avoided).

Each graph is a `defaultdict(list)` of `(neighbor, weight)`. The
weight is `edge.routing_weight_mm or edge.distance_mm`.

In short, **PathRouter's first act is "classify topology edges into
four policy-specific adjacency lists simultaneously"**. Each `find_*()`
call picks the appropriate graph and runs Dijkstra.

## Context

`policy/routing/router.py` performs two responsibilities together:

- **Naming**: it is the sole owner of the topology naming convention
  (`sip{S}.cube{C}.<comp>`, `sip{S}.io{I}.pcie_ep`, etc.). Components /
  probe / IPCQ install / runtime API do not build node-id strings
  themselves — they call helpers.
- **Path decisions**: policy separation by `edge.kind`. For the same
  src→dst, different routing intents (PE DMA vs M_CPU DMA vs general
  component routing) call for different adjacencies and so produce
  different paths.

This helper API is widely consumed (probe.py / distributed.py /
install.py / various components / tests), yet **the exact signatures /
return semantics / adjacency picks** are not gathered in any ADR. This
ADR closes that gap.

## Decision

### D1. `AddressResolver` exposes five public methods

#### D1.1. `resolve(addr: PhysAddr) -> str`

Translates a `PhysAddr` to a destination node id in the topology:

```
addr.kind == "hbm"             → f"sip{s}.cube{d}.hbm_ctrl.pe{pe_id}"
  where pe_id = addr.hbm_offset // self._hbm_slice_bytes  (ADR-0017 D4/D9)

addr.kind == "pe_resource":
  addr.unit_type == PE         → f"sip{s}.cube{d}.pe{addr.pe_id}.pe_tcm"
  addr.unit_type == SRAM       → f"sip{s}.cube{d}.sram"
  addr.unit_type == MCPU       → f"sip{s}.cube{d}.m_cpu"
  others                       → RoutingError("unsupported unit_type")

other kinds                    → RoutingError("unsupported address kind")
```

If the derived node id is not in `self._node_ids`, raises
`RoutingError(f"node {node_id} not found in topology")`. So even when
the address has valid syntax, an absent node in the topology
fails-loud.

#### D1.2. `find_m_cpu(sip, cube) -> str`

Returns `f"sip{sip}.cube{cube}.m_cpu"`; absent → `RoutingError`.

#### D1.3. `find_pcie_ep(sip, io_id="io0") -> str`

Returns `f"sip{sip}.{io_id}.pcie_ep"`; absent → `RoutingError`.

#### D1.4. `find_io_cpu(sip, io_id="io0") -> str`

Returns `f"sip{sip}.{io_id}.io_cpu"`; absent → `RoutingError`.

#### D1.5. `find_all_pcie_eps() -> list[str]`

All PCIE_EP node ids across all SIPs, sorted. Filtered by
`endswith(".pcie_ep")`. Cross-SIP IPCQ uses this when enumerating
PCIE_EPs.

This class is the sole owner of the naming convention
(`sip{S}.cube{C}.<comp>`, `sip{S}.{io_id}.<comp>`) — ADR-0015 D4.
The topology builder produces nodes with the same naming convention;
components never build node-id strings directly — they go through
these helpers.

### D2. `PathRouter`'s four adjacency graphs

Constructed in one pass. `edge.kind` drives policy:

| graph             | excluded edge kinds                                                                                                                   | use case                                       |
|-------------------|--------------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------|
| `_adj_all`        | (none)                                                                                                                               | M_CPU↔NOC command included, IO_CPU/M_CPU routes |
| `_adj`            | `"command"`                                                                                                                          | PE DMA / generic data paths                     |
| `_adj_mcpu_dma`   | `"pe_internal"`, `"pe_to_router"`                                                                                                    | M_CPU DMA (skips PE pipeline)                   |
| `_adj_local`      | `_UCIE_KINDS` (`ucie_internal`, `ucie_conn_to_router`, `router_to_ucie_conn`, `ucie_conn_to_noc`, `noc_to_ucie_conn`, `ucie_mesh`, `io_to_cube`, `cube_to_io`) | same-cube routing (UCIe bus excluded) |

Each graph is `dict[node_id, list[(neighbor, weight)]]` with weight =
`edge.routing_weight_mm or edge.distance_mm`. Excluding command edges
prevents them from influencing routing; isolating `_adj_local` keeps
UCIe's "zero-distance bus" from out-competing the mesh — consistent
with ADR-0017 D7's cross-PE-slice mesh-distance requirement.

### D3. `PathRouter` exposes six public methods (+ two backward-compat shims)

#### D3.1. `find_path(src_pe: str, dst_node: str) -> list[str]`

**PE DMA routing**. `src_pe` is a PE prefix (e.g.,
`"sip0.cube0.pe0"`); the function auto-prepends `.pe_dma`, making the
true start node `"sip0.cube0.pe0.pe_dma"`.

Adjacency depends on cube-locality (`_same_cube`):

- **Same-cube** (src and dst share `sip{S}.cube{C}.` prefix): uses
  `_adj_local`. Excluding UCIe lets cross-PE-slice access pay accurate
  mesh distance (ADR-0017 D7).
- **Cross-cube**: uses `_adj`. UCIe naturally becomes the right choice
  for the cross-cube portion.

#### D3.2. `find_path_with_distance(src_pe, dst_node) -> tuple[list[str], float]`

Same adjacency policy as D3.1, but returns `(path, total_distance)`.
Used by probe and analysis tools that need the distance metric.

#### D3.3. `find_mcpu_dma_path(m_cpu_id: str, dst_hbm_id: str) -> list[str]`

**M_CPU DMA path**. Same cube → `_adj_local` (stay within the mesh);
different cube → `_adj_all` (cross via UCIe). The
`_MCPU_DMA_EXCLUDE` set ensures PE-pipeline nodes never appear on
M_CPU's routes.

#### D3.4. `find_memory_path(src: str, dst: str) -> list[str]`

Direct memory path like
`pcie_ep → io_noc → cube → router mesh → hbm_ctrl`. Uses
`_adj_mcpu_dma` to exclude `pe_internal` and `pe_to_router`, so
host-issued reads/writes never leak into the PE pipeline. Probe
(ADR-0049 D1's H2D/D2H cases) calls this directly.

#### D3.5. `find_node_path(src: str, dst: str) -> list[str]`

Generic routing between arbitrary nodes, **including command edges**
(via `_adj_all`). IoCpuComponent / MCpuComponent use this when they
need to route through M_CPU ↔ NOC command-kind links.

#### D3.6. Backward-compat shims

- `_dijkstra(start, goal) -> list[str]` — thin wrapper for
  `_run_dijkstra(self._adj, …)`.
- `_dijkstra_with_dist(start, goal) -> tuple[list[str], float]` —
  distance-aware variant.

Despite the underscore prefixes (suggesting internal API), existing
tests call these directly. New code should prefer D3.1–D3.5; these two
shims are deprecation candidates.

### D4. Dijkstra — single-source shortest path

`_run_dijkstra_with_dist(adj, start, goal)`:

- `heapq` priority queue.
- `best: dict[node, distance]` — best known distance to each node.
- `prev: dict[node, predecessor]` — for path reconstruction.
- Edge weight = `routing_weight_mm or distance_mm`. The separation
  matters because UCIe (and a few others) declare an explicit
  `routing_weight_mm` distinct from physical `distance_mm`.

`start == goal` short-circuits to `([start], 0.0)`. Unreachable target
→ `RoutingError(f"no path from {start} to {goal}")`.

The algorithm is **deterministic**: identical graph + start/goal gives
the same path, satisfying SPEC R1 ("routing MUST be deterministic").
Tie-breaks follow `heapq`'s push order (Python list order is
deterministic).

### D5. Single-owner principle for helper-API decisions

The following decisions live only inside router.py:

- Naming convention: `sip{S}.cube{C}.<comp>`,
  `sip{S}.{io_id}.<comp>`,
  `sip{S}.cube{C}.hbm_ctrl.pe{pe_id}`.
- Adjacency policy: which edge kinds belong to which graph.
- Algorithm for recovering PE id from an HBM slice size.
- Dijkstra weight selection
  (`routing_weight_mm or distance_mm`).

Breaking single ownership (e.g., a component starting to build
`f"sip{s}..."` itself) would explode the blast radius of naming-
convention changes. This aligns with ADR-0015 D4.

### D6. Consumers of the helper API

Methods listed in this ADR are called from (current corpus):

- `probes/probe.py` (ADR-0049): `find_pcie_ep`, `find_io_cpu`,
  `find_m_cpu`, `find_node_path`, `find_mcpu_dma_path`,
  `find_memory_path`, `find_path`, `resolve`.
- `runtime_api/distributed.py` (ADR-0047): indirectly (engine-internal
  routing).
- `ccl/install.py` (ADR-0023): `find_all_pcie_eps`, `resolve`.
- `sim_engine/event_log.py`: like probe — `find_pcie_ep`,
  `find_memory_path`.
- `components/builtin/m_cpu.py`, `components/builtin/io_cpu.py`:
  `find_node_path`, `find_mcpu_dma_path`.
- Tests (test_routing.py, test_cross_sip_routing.py, …): most of
  D3.1–D3.5.

When a new consumer arrives, D1/D3 act as a first-pass guide on
whether an existing method matches the intent or a new one is needed.

## Alternatives Considered

### A1. One adjacency graph + per-call edge-kind filtering

Rejected. Re-filtering the graph on every `find_*()` call hurts
Dijkstra cache locality. Constructing four graphs in one pass (D2)
has modest memory cost (edges ≤ a few × 10⁴), and selection happens
in O(1) at call time.

### A2. Drive adjacency separation by separate edge metadata rather than `kind`

Rejected. `edge.kind` is already assigned by the topology builder
(ADR-0015 D4 + ADR-0017); a parallel metadata field would force
synchronization between two systems.

### A3. Use BFS with uniform weights instead of Dijkstra

Rejected. With per-edge `routing_weight_mm` (mesh link / UCIe /
IO-internal), BFS minimizes hop count rather than total
latency/distance. SPEC R1 + R2 require deterministic and accurate
routing, which BFS does not deliver.

### A4. Express the helper API as module functions instead of classes

Rejected. Each class
(`AddressResolver`, `PathRouter`) maintains caches
(`_node_ids`, `_hbm_slice_bytes`, four adjacency graphs) reused across
many routing queries on the same graph. Module functions would have
to rebuild state per call or go global, hurting safety and
performance.

## Consequences

- When components / probe / IPCQ install / runtime API all go through
  router.py helpers, a naming-convention change (e.g., `.io0.` →
  `.iochiplet0.`) is a one-file edit (D5).
- D2's four-graph split is now ADR-locked, so when a new edge kind is
  added (e.g., a new inter-die UCIe-link kind), the right adjacency
  category is decided explicitly rather than by default.
- D3.1's same-cube vs cross-cube branching (ADR-0017 D7) is explicit,
  so anyone changing routing knows which adjacency to touch.
- D6's consumer list bounds PR-review scope for helper-API changes,
  and the backward-compat shims (D3.6) are flagged as deprecation
  candidates.